Sewage treatment process



Oct, 21, 1941. A. L. GENTER SEWAGE yTREATMENT PROCESS Filed March 8;.,1939 2 Sheets-Sheet 1 0d. 21, 1941. @ENTER r 2,259,688

' sEwAGE TREATMENT PRocEss FiTedl March 8, 1959 sheets-sheet 2 v posalof this liquor diincult problem.

Until ,the present invention, it has been as UNITED STATES PATENT l oFFIca l l snwAGa 'mgm rnoonss' .Albert n center, Bauman, Ma. ApplicationMaren s, 1939, serial No. 260,654

12 Claims.' (Cl. 21d-2) My invention relates to sewage treatment and l:

has particular application to, (a) the treatment of the more liquidportion or supernatant liquor obtained during the anaerobic digestion ofthe various sewage sludges accumulated during the 'sedimentation processin primary and secondary treatment of sewage, and (blthe accumulationvand disposal of the finely dispersed solids suspended in thissupernatant liquor.

Elutriation, as shown by my Patent No. 1,999,973, has been extensivelyused and has become standard practice in many large cities, such asWashington, Hartford, Baltimore, San Francisco, Greensboro, andWinnipeg, Manitoba; Notwithstandingthis extensive use and the generalknowledge ofthe art with respect to elutriation,

no one has heretofore appreciated that the e1u y triation process may beused to solve theproblem whichhas heretofore existed in respect to the Il disposal of the supernatant liquor from sludge digestim TheSatisfactory treatment and dis sumed that the problem of disposing -ofthis liqhas for years presented` a uor could not be satisfactorily `ridof its difficulty, and the solution generally adopted was the rel turnof the liquor tothe primary treatment.

The present invention is based on the discovery that contrary to this'assumption that the liquor may be'properly returned to the primarytreatment, this supernatant digestedliquor contains a substantial amountof suspended solids which decrease the efciency of the primary sewagevtreatment process, and also upon the discovery that by elutrlatlonof'the suspended solids in this .liquor, as well as the digested sludge.the emlected in the secondary or final tanks is termed secondary or nalsludge.' The settled voluminous sludges drained or removed from thebottoms of such primary and secondary tanks are then treated anddisposed of by varlousmethods. Wherever secondary o r nal sludge isproduced and collected, it is generally mixed with the 'pri- Amarysludge in theprimary settling tank before further treatment and disposalof the collected mixture.

In order to better control the odor nuisance which can result fromarapid haphazard decomposition or putrefaction ofsuch sludges, a largenumber of modern sewage treatment plants subject the sludges to furthertreatment `in a specially cultivated bacterial environment which ar--rests the activities of odor producing organisms and decomposes themajor portion of the organic matter in the sludges with remarkablylittle odor nuisance. 'I'his scientically controlled decompositionprocess is known as sludge digestion,

which destroys about half of the organic matter present, thus materiallyreducing vthe amoimt of'4 l sludge, and producing a stable non-offensivedigested sludge of increased mineral content.

Aside from this the digestion process produces relatively largequantities of sludge gas or diges-A tion-gas containing methane, carbondioxide, ni-

f trogen and some hydrogen, which is of considerciency ofany treatment'system may be substantially increased.'

All modern sewage treatment plants involveequipment for the removal of alarge percentage vof the suspended settleabl organicsolids as faraseconomicallyposslble from the large amount of polluted water.Thisremoval is effected by sedimentation of the settling solids,preferably' 4 in mechanically operated sedimentation or settling tanks,called primary settling tanks, thus facilitating further chemical vorbiochemical treatment of the clearer enluent, called settled sewage,from these tanks. This eilluent may re` ceive no further treatment ormay be subjectedl to lfurther chemical. or vbiochemical oxidationprecipitation of further colloidal masses of organic solid1llquidcomplexes. The-collection 8f able value as fuel and sourcel of power. Yv

In moderny treatment practice the principal objects of digestion are:(1) the rapid production of a stable digested sludge, (2) regularproduction of such sludge, (3) satisfactory separation of digestedsolids from the surrounding sludge water or moisture, known as sludgeliquor 0r sludge f treatment whicnresultsin the lflocculation or 55supernatant, (.4) production and collection of the maximum amount ofsludge gas, and (lfreedom from evolution of objectionable odors.

Asvthe primary or mixed primaryandsecondary sludges going to sludgedigestion may contain'from 90 to 98 percent water and the organic solidsin these sludges aredecomposed, less solids `result in digestion thusincreasing the percentage of water present with the result that thedigested or partially digested solids must again be settled to a new andmore stable sludge (digested sludge) on the one hand anda supernatorapid digestion, by

' heating the digestion most of the evolved tant liquid, called sludgeliquor or supernatant liquor, on the other hand, as has been indicatedin the preceding paragraph.

The degree to which sludge digestion is carried depends on severalfactors.4 If the sludge gas is required for heating and powerproduction, or the sludge is to be dried on sand beds, pro-V longeddigestion periods at constant temperature are advised in order to insurea maximum yield of gas and a maximum reduction in organic solids-whichproduces a relatively odorless sludge of lhigh solid content forsand beddewatering.

However, if the digested sludge is to be dewatered in mechanicalequipment according to my U. S. Letters Patent, No. 1,999,973, and driedand incinerated, shorter periods of digestion can be used. Although thegas production may be somewhat diminished by this rapid digestion, theprocess of sludge elutriatlonV insures a sludge and filter cakeremarkably free of odors.

Modern digestion tanks are built as separate tanks operating incommotion with the sludge sedimentation and collecting tanks. They aremostly circular in shape, generally constructed of concrete, equippedwith rigid or floating covers, with and without stirring mechanisms,inlets for introduction of primary and secondary sludge, bottom outletsfor withdrawing digested sludge suspendedsludge particles. Near and, atvarious tank levels, outlets for withdrawing sludge liquor orsupernatant liquor. Gas collecting domes are also provided with moderncovered digestion tanks so that the collected gas may be removed fromthe tanks by piping for use in heating and power generating equipment.Sampling cocks are also provided for observation and sampling of gestedsludge and sludge liquor or supernatant liquor. vSuch modern. arelatively heating equipment which the introduced sludge, di-

tanks are maintained at uniform temperature, most conducive utilizes aportion of the sludge gas for fuel and heat transfer through water whichis properly circulated in heating coils arranged within the digestiontanks. Asv there is more sludge-gas produced than required for tankheating purposes, some modern plants use the gas for power production ingas engines. In this case the cool. ing water from the `gas engines, andeven the heat from the engine exhaust gases, are used for tanks. Theexcess unused sludge gas is wasted to the atmosphere by burning inespecially protected devices. All of this equipment also requires propermetering for sludges, gas and sometimes for supernatant liquor.

In separate provision is made for gas collection and utilization and formaintaining optimal digestion temperatures, with the result that thedigestion space required is materially larger than with modernequipment. Such older tanks are usually open totheatmosphere. 4 v.

As the .organic sludge particles decompose through anaerobic action indigestion tanks these particles evolve gas amounting to approximately 15cubic feet per pound of volatile organic matter digestion tanks of olderdesign no equipment considerable expensive present in the sludge. 'Thedigesting particles therefore rapidly disintegrate and tend to float orremain in suspension in the surrounding water or liquor, due to theflotation effect of evolving and entrained or imbibed gas bubbles. Asthe gas'.

evolution practically ceases, digestion similarly approachescompletion.- Thenthe particles lose gas andtend to sink to the bottom ofthe digestion tank together with the ter and fiocculent massesof'digested, but not completely decomposed, organic sludge.

'I'he water or solution rich in dissolved decomposition productsassociated with the sludge solids is termed sludge liquor. Therefore thesludge proper consists of the sludge solids and this liquor. The liquidcontents of any digestion tank above the bed or blanket of bottomsludge, usually contains less total solids after evaporation than doesthe sludge proper and is usually termed either "supernatan supernatantliquor or sludge liquor. The sanitary engineer uses these termsinterchangeably for the same thing. In the foregoing and 4following theyare likewise so used. f

During active digestion, the decomposing solids will repeatedly floatand sink until gas evolution ceases, thus mixing the tank contents andcausing the upper, somewhat lighter, liquor portion to become quiteblack and dirty with finely divided the finish of the digestion periodthe tank contents become more quiescent thus permitting most of thedigested solids to settle more readily, which leaves a somewhat -clareryellowish to black liquid portion always referred to as supernatantliquor. The gas evolution and flotation .process leaves some of thesolids enmeshed in a flotation froth, called scum, which collects at theupper liquid level of the tank'contents. If this at frequent intervals,some of the oated solids are freed of gas' and settle, thus again makingthe supernatant dirtier.

In order to insure a gested ,sludge particles from the supernatantliquor, multi-stage digestion has been introduced. This consists in theuse of a system of two or more digestion-tanks in succession, eachconnected with the other for the progressive and automatic transfer ofsludge and gas during digestion. 'Ihe primary digestion tank is used forkeeping the sludge during active digestion and the digesting contentsare kept'at a constant level. In some cases mechanically operatedstirrers are provided in these primary digestion tanks to thoroughly mixthe tank contents periodically 'and to thereby somewhat hasten thedigestion process. In the s ucceedingtankortanksthe'digesting mixturerests under moreA quiescent` conditions thus allowing the digestedsolids to settle away from the sludge liquor to form supernatant liquor.From to 90 percent of the digestion process takes place in the ilrststage or primary digestion tank.

It will be seen that even with modern digestion tank space for sludgedigestion and separation, of sludge and supernatant mustbe provided, i.e. from 1 to 4 cubic feet of space per capita served. If but one cubicfoot of space is provided, digestion will be pushed to a relatively highrate in which case the contents of the digestion tank or tanks will bekept in a turmoil with the result that the Vsupernatant -liquor willcontain a high proportion scum is broken down l better separation ofdiin a homogeneous condition' to filtration increases with the thicknessof the cake. Hence, yto get three times as much filtrate through thesame ultimate cake thickness in one instance than in another, at thesame pressure, can take three times or more longer. or if the drainingis done on a greater area, three times as much area producing one thirdthe cake thickness will be required. In cases Where digested sludges arepartially dewatered in mechanically operated continuous suction filters,it has'been suggested that the total mixture of sludge and supernatant-be chemically precipitated with lime and ferrie salts and theprecipitate containing the sludge solids be drained in such lters. Thisis unfeasible from a standpoint of economy, be-

cause (a) the successful operation of such filters requires that thesludge cake be not less than about 11g inch in thickness in order toprovide sufficient matted cake material during each lter drum revolutionto insure discharging the entire cake as a uniform sheet during the cakedis' charge period. Thin strips lof cake, even though .relatively dry,which fail to discharge and are left y#son the revolving drum rapidlysmear and blind the filter medium, thus materially diminishing theavailable filter area, if not blinding thev entire drum; (b) to gain thenecessary filter yield in daily sludge solids considerably more lterarea and auxiliary Aequipment are necessary, which means a proportionateincrease in power, labor vand maintenance per ton of sludge solidsdewatered; and (c) as I ha've shown in my U. S.

pound of sludge solids in No. 1 to produce No. 2, the chemical savingsper ton of sludge solids treated were reduced by about $4.45 from $7.25j or about 61 percent, showing that the amount and cost of precipitatingchemicals used is roughly a function of the amount of sludge liquorpresent in the sludge as moisture.-

3.-Whe'n more of the original sludge liquor present as moisture wasremoved,.i. e. 4about 63r percent of the moisturev remaining in No. 2 orabout 89 percent of that present in No. 1 by'displacing most of thissludge liquor -with purer water through simple single stage elutriationaccording to my Patent No. 1,999,973, the following remarkable reductionin chemicals took place. Using three volumes o'f purer water forelutriation purposes and allowing the washed sludge to settle in asingle tank, a sludge having 6 percent solids and 94 percent purerliquid moisture resulted. Adding to this washed sludge but 3 percentferrie chloride on sludge solids and no lime resulted in a filteryield-A of 4.5 pounds of dry sludge solids per square foot of filterarea per hour'at a cost of $1.20 per ton of sludge solids for the ferricchloride. This was $6.45 or 84 percent less than the $7.25 chemical costunder No. 1

and $1.60 less or 57 percent less than the $2.80

Letters Patent No.- 1,999,973 and practice has demonstrated in all caseswhere thevacuum filtration of digested sludges is used, withoutsubjecting the sludges to the elutriation treatment outlined in thispatent, the amount of precipitating and coagulating chemicals necessaryto render sludge solids granular enough to permit ready draining in suchfilters is roughly a function of the soluble decomposition productspresent in the sludge liquor, or moisture, and consequently in thesludge supernatant, This likewise means that the amount of precipitatingchemicals used is roughly a function of the total amount of moisturepresent as sludge liquor or supernatant in any digested sludge. The moreof this moisture or supernatant liquor there is associated with thesludge solids, the greater will be the amount of lime and ferriechloride used to precipitate the dissolved decomposition products and torender the sludge solids drainable in any continuous vacuum filter.Consequently, the greater will be the chemical operating costs per tonof sludge solids in the cake yield of the filter.

For example: No. l-a dilute digested sludge containing but 1-.5 percentsolids (98.5 percent supernatant as moisture) required 9.7 percent-ferric chloride and 33.7 percent lime, costing about $7.25, per ton ofsludge cake solids yielded by the filter, to produce but one pound ofsludge solids per square 4foot of filter area per hour as y filteryield. This one pound yield corresponded to about 63 pounds of filtrate.Also, No. 2-When the sludge moisture in this same sludge was firstdiminished by proper sedimentation, so that 5.5 percent solids and 94.5percent sludge liquor moisture remained, alter yield of 3.5 pounds persquare foot of filter area per hour and about 20 pounds of filtrateresulted from using 3.5 percentuferric chloride and 14 percent lime,costing about $2.80 per ton of cake solids yielded by the lter. Thismeans that 'in removing about 70 cost per ton of sludge solids under No.2.

It is, therefore, evident that dewatering the total mixture of digestedsupernatant and digested sludge on vacuum filters materially increasesthe amount of detrimental sludge liquor present and materially increasesthe filtration costs while producing much lower filter yields. In rderto precipitate all of the soluble decomposition products present in thesludge liquor and supernatant and at the same time render the V sludgesolids coarsely coagulated so as to permit free and relatively rapidfiltration, relatively enormous quantities of lime and ferric salts haveto beadded to the mixture, in which case the 'lter cake is largelycomposed of chemical pre- It is now a well known and widely published fY fact that sludges elutriated .according to my Patent 1,999,973 showmaterially greater filter yields without the use of any lime and at verylow additions of ferric or aluminum salts than can beobtained when thedetrimental sludge liquor moisture is not displacedby purer water. Inusing this process for digested sludges I have also discovered thefollowing facts relating to the displacement of the supernatant ,liquorfrom the suspended solids containedv in same by purer water through theprocess of elutriation. These discoveries relate. (a) to the-elutriation of the supernatant solids and (b) the solids in mixtures ofsupernatant liquor and digested sludge:

(a) I have discovered that treating the supernatant liquor containingvsuspended solids by elutriation, i. e. diluting and mixing it with waterrelatively free of dissolved decomposition products, permitting thesolids to settle to a sludge.v anddecanting the diluted supernatant inan elutriation settling tank not used for primary or secondary sludgesettling, accomplishes a more rapid separation of the suspended solidsfrom the -M dilute liquor than takes place'if the suspended escapes fromthe mixture.

the suspended solids remaining in the liquor will amount to an averageof aboutI 2000 parts per million. f

As previously indicated, the fresh sludges introduced to digestion tanksare largely composed of water, i. e. 98 to 90 percent, the proportion ofwater to solids really increases during the digestion process, becausethe organic solids decompose largely to gas, some of which concentratesto saturation in the sludge liquor and most of which 'I'hisdecomposition also produces ammoniacal and other compounds which combinewith some of the evolved carbon dioxide 'to form soluble ammoniaca]carbonates, bicarbonates, carbamates, etc., which accumulate in and foulthe sludge liquor as soluble decomposition products. Due tothis-increase in the proportion of Water to decomposed and mineralizedinsolubles during digestion the remaining insoluble solids-are againconcentrated to produce the digested sludge in the digestion tanksprovided.

'I'he sludge liquor or supernatant liquor produced by this procedureamounts to from 2 to slightly over 3 gallons per pound of suspendedsolids introduced to the digestion tank from primary and secondarysewage settling tanks. 'I'hus in a small plant treating one milliongallons of sewage daily from ten thousand inhabitants, the solidsintroduced to digestion from Y primary sedimentation alone may amount toabout 1000 pounds (1/2 ton)l,daily and the .di-

gested sludge supernatant may be from 2000 to 3000 gallons (B1/2 to121/2' tons) daily. If the plant has complete treatment, i. e.purification by activation, the solids introduced to digestion mayamount to 1600 pounds daily in which case the supernatant liquor canamount to 4800gallons (20 tons) daily. If this supernatant is'relativelyclean, it can contain about 100 pounds or more daily of suspendeddigested sludge particles for every ten thousand population served. Insome cases, the ligure is four or five times this amount.

Therefore, the third objective of sludge digestion, listed in theforegoing, i. e. the satisfactory separation of digested solids fromsupernatant f liquor, is exceedingly diiiicult to obtain in moderndigestion practice, especially where rapid digestion is resorted to, andseveral suggestions have been made and tried in the effort to denitelysolve this problem.

Usuallythe supernatant liquor is returned to4 the sewage-treatmentprocess by Way of the primary settling tanks. Where the digested sludgeis dewatered and partially dried on sand beds, the supernatant liquor,if particularly dirty, is sometimes run onto the sand beds also.Attempts have also been made to run this liquor 'into special settlingtanks to permit more of the suspended solids to settle. However, if thissettling does not occur in the large digestion tanks,

`especially final stage digestion tanks of multistage digestion, thereis little or nothing to be gained by such expensive settling treatment.As previously indicated, the suspended solids are usually floated orheld in suspension because (a) they contain adsorbed or imbibed.decomposition gas,. and (b) :the surrounding sludge liquor is saturatedwith the same gases and partially saturated with other solubledecomposition products thus resisting the'escaping tendency of the gasesheld by the remaining suspended sludge particles. As a consequence ofthese facts, most of the suspended solids present in the super- 'pocketsof clearer liquor. In withdrawing supernatant at various tank levels,these floating sludge banks can also be withdrawn, thus producingdecidedly dirty eilluent.

The main reasons for returning as clear a supernatant liquor as possibleto the primary settling tanks is to prevent unnecessary and troublesomeaccumulation of decomposed or partially decomposed organic solids inthese settling tanks designed to settle out the fresher solids beingcarried by the sewage stream entering the plant. The' return ofpartially digested solids in dirty supernatant liquor to this primarysettling system not only increases the amount of sludge accumulatedinthe primary settling tanks, but also tends to make this sludgelighter, thus taking up settling space. Furthermore, it tends tointroduce more anaerobic decomposition organismsf, thus increasingdecomposition with gas evolution in the primary tanks. This can have theeffect of making the primary sludge lighter and more voluminous, thusdecreasing the eective settling space and producing a thinner sludgegoing to digestion, which in turn overloads this valuable space withunnecessary water. 'I'he combined eiect is then to produce a circulatingload of partially digested solids returning in still dirtier supernatantfrom digestion back to primary settling and again back to digestion withthe fresh primary sludge. In addition to this, any undue accumulation ofsupernatant solids in the primary .settling tanks tends to produce adirtier overilow from these tanks, whereby the biochemical oxygen demandof the primary tank overflow may be increased, thus lessening theeillciency of these tanks.

As the percolating and sludge drying capacity of any sand lter beddepends, among other things, on the proportion of the sludge retained bythe bed to clear liquor which drains through the sludge an'd sand in anygiven time, conveying the entire mixture of digested sludge andsupernatant onto sand filters can and does require liquor associatedwith one ton of digested sludge solidswere run onto a sand bed toproduce a final sludge cake containing 60 percent moisture,` the amountof water to be drained and evaporated by the sun could be 29 tons.Whereas, if 20 tons of supernatant were first 'removed and the sludgecontaining the remaining 1800 pounds of digested solids .were put onsand illters to produce a sludge cake of 60 percent moisture, therewould be b ut about 9.2 tons of water to be drained, which is but athird oi' the amount drained from the entire mixture of supernatant andsludge. Draining 29 tons of water could require three times or more vofthe sand bed area than draining 9.2 tons for the simple reasons thatfiltration rates decrease not only with decrease in the size of theparticles forming the cake (the sludge particles in the supernatant arematerially ner than those in the sludge) but, what is of equalimportance, the resistance of any accumulating sludge cake the suspendedsolids than has heretofore been accomplished, thus permitting the returnof less suspended solids from the digestion tanks to the primarysettling tanks in the: sewage treatment process. The settled elutriatedsolids then may be collected in a small amount of sludge and conductedfrom the separate elutriation treatment system by way of the sludgedewatering equipment without circulating back to primary sedimentationand again to sludge digestion and so Y supernatant liquor and dirtiersludge liquor with water which is relatively free of dissolved gases anddissolved decomposition products as the first step in my elutriationprocess, most of the i suspended solids yield their attached gases tothepurer water, thus becoming heavier so that they settle far morerapidly than if left in their natural supernatant liquor. The remainingcolloidally -dispersed particles may be precipitated and swept down byadsorption during elutriation if necessary, as will be hereinaftershown. Added sulting supernatant. This necessary and importantseparationvcan be accomplished in the y' materially smaller and lessexpensive vtank or tanks installed and employed for sludge elutriation.In this case, if the heavier sludge normally depositing in the firststage digestion tank can be run onto sand beds for drying, the verydirtyv i supernatant can be elutriated and the suspended solidscollected as elutriated sludge can then be* mixed with the heaviersludge from the digestion tank or tanks and dewatered together on thesand` beds, while the relatively clear overflow from the elutriationprocess is returned to the sewage treatment process.

's I have shown, most of the nrieiy divided to this removal of imbibedgases from the suspended sludge particles by solution in a purer liquidmedium is the fact that diluting tl-le supernatant liquor with purerwater diminishes the concentration of soluble decomposition products inthe supernatant liquor, thus making it somewhat lighter which againfacilitates the settlement-of most of the suspended solids. Experimentsmade on supernatant liquor from l a digestion tank'used for digestingprimary and waste activated sludges show that by diluting this liquor inthree volumes of relatively pure water (purified treatment planteiiluent) most of the suspended solids settle from forty to fifty times`faster in the dilute mixture than in the super-v natant itself' andproduce a sludge containing a higher percentage of solids, because thesettled washed solids contain less imbibed gases on the one hand andsettle in a liquid mixture of lighter gravity on the other hand. Thus,the elutriation of the *suspended solids in sludge liquor or super- Ifall digested and partially digested solids and associated moisture inthe sludge containing the former are to be Apartially separated from thesludge moisture in mechanically operated filters, both the digestedsludge normally depositingin the rst stage digestion tank and the dirtysuper# natant can pass through the elutriation process, wherein theheavy washed sludge proceeds to coagulation and filtration while theclearer supernatant overflow (elutriate) lfrom the elutriation tanks isreturned'to the treatment process,

In elutriating the supernatant liquor alone, a mixing device forthoroughly mixing the supernatant and elutriating water followed by asingle settling or thickening `tank is sometimes'sunicient. Thiselutriating settling tank will be considerably smaller than the primaryor secondary stage digestion tanks employed in modern plants because ofthe greater settling rates of the suspended solids in the mixtures ofpurer water and supernatant liquor. For example, if the second or anysubsequent stage digestion tank is de.

signed to have but the low capacity of one cubic foot of space percapita for the digestion of primary sludge and two cubic feet percapita, for the ydigestion of mixed primary and secondary elutriationtank space will be but 0.07 cubic foot natant liquor can be of definiteeconomic ad- Vantage in effecting a 'better separation of the clearliquor from the suspended solids and can thereby become a material aidin accomplishing one of the principal objects of the sludge digestionprocess.

(b) As a result of the facts disclosed in fore-- going discovery (a),the total mixtureof digested sludge solids and sludge liquor whichbecomes supernatant does not have to be separated by sedimentation insecond 'and subsequent stage digestion tanks to form sludge andsupernatant thus leaving unnecessary amounts of digested and partiallyldigested solids in suspension in' the reslightly more elutriationquired than for elutriating the digested sludge per capita if used forv8 hours daily and 5 day weekly service and about one third of this spaceif used continuously. In other words, the elutriation of the supernatantliquor reduces the'lspace necessary for separating the supernatantliquor from the suspended sludge. solids approximately percent from theone or two cubic foot per capita space used mainly for concentration ofsupernatant solids in digestion tanks.

vIn elutriating the total mixture of sludge and second or subsequentstage supernatant from first or' second stage digestion with the view ofthoroughly washing all of the solids and dewatering same in mechanicalltering equipment while returning a clearer supernatant elutriate to thesewage treatment process,

alone or for elutriating the supernatant liquor alone, as there will bemore elutriating water used to displace more ytotal sludge liquor fromthe total mixture of digested sludge and its supernatant. Oneelutriation tank triated sludge can be used for smaller installationsand two elutriations tanks will be better for larger installations inorder to permit thorough washing of the total mixture by countercurrentor two stage elutriationprocedure. In any case, the total elutriationtank space for tank space will be rewith rewashing of the elustage.f orstages. nal stage digestion' watering service and about one third ofthisspace for continuous service, according to whether only primary or amixture of primary and secondary sludges are digested. This againresults in a very material saving in second or subsequent stagedigestion tank space.

In any of these procedures the sludge and supernatant liquor of thetotal mixture of both can be drawn together from the digestion tank ortanks and elutriated, or the sludge -and supernatant can beYindependently drawn at independent intervals and treated separately atindependent intervals. Or again one may desire to elutriate buta portionof the supernatant that is dirtier than the balance. This portion may beelutrlated with or without the digested sludge.

If two or more stage digestion tanks are already installed and in useand the elutriation system is subsequently installed, sludge may bedrawn from the second or final stage digestion tank together` with thedirty mixture of super natant'and partially digested sludge solids fromthe flrst or preceding stage digestion tank or tanks and elutriated. Orthis sludge may be drawn together with clearer supernatant from thefinal stage digestion tank or tanks and elutriated. Anylfof theseprocedures will be of ad vantage in providing more digestion space atany desired time. Excessive expensive digestion space is frequentlyinstalled in order-to provide for ample storage space in case ofperiodic disturbances in digestion and in case the peak loads of primarysludge become unexpectedly excessive. The foregoing provisionseliminate-the necessity of providing so much excessive digestion space.

Furthermore, where two or more digestion tank so that its formerly usedheating system should be disconnected in case heating coils or other hotwater connections are present. If heat were applied by the direct orindirect application of hot water to such a tank or any tank being usedfor elutriation settling purposes, sludge digestion would again takeplace thus defeatingthe purpose of elutriation renoval of decompositionvproducts and settling of the elutriated sludge solids.

(c) The solids usually suspended in the digestor supernatant liquor arenot only more nely dispersed than in the settled sludge, but are alsoless digested, meaning that the suspended solids and especially thosecollecting in the scum may contain more or less organic volatile matterthan those in the sludge and continue evolving some gas. I havediscovered that these suspended and scum solids are more difficult totreat with chemicals and drain free of moisture in continuous vacuumfilters than are the settled more completely digested solids collectedin the bottom digested sludge of digestion tanks. This is particularlytrue of the suspended solids in the thinner dirtier supernatant mixtureof liquor and partially digested solids in the first stage of modernheated digestion tanks, where completer separation of clearersupernatant and digested sludge particles has not taken place. In orderfor such rst stage digestion tank solids to become better adapted toeconomic treatment with lime and ferric salts for vacuum filtration suchsupernatant solids from the first digestion stage tanks are installedand used in series or stages y and funds are not available for theinstallation of elutriation settling space, one of the digestion tankscan be used for this purpose. As the space -occupied by any singledigestion tank is far in excess of that necessary for elutriationsettling, either. copious amounts of water can be used or elutriatedsludge solids can be accumulated and stored over several days time aselutriated sludge which can be re-elutriated by recirculation withelutriating Water back into the same tank. In

such an arrangementV the supernatant and/or mixtures of same with sludgefrom the preceding digestion tank or tanks must be withdrawn from thepreceding tank system, thoroughly mixed with elutriating water andintroduced to the dlgestion tank being. used for elutriation settling.

In this case a constant overow level and convstant overflow ofsupernatant elutriate during elutriation can be provided. Furthermore,as such a tank provides copious storage space for the settled elutriatedsludge provision can be made for recirculating the stored washed sludgeback through this tank with elutriatlng water to insure thorough washingin case such is desired. In using a nal stage digestion tank f or suchelutriation purposes it ceases to function as a digestion tank andbecomes an elutriation sety tling tank. Therefore, this tank should beisolated` from the foregoing digestion stage or stages so that no airgains access to the gas co1- lecting system and conduits of theforegoing lFurthermore, the use of any tank for elutriation purposeseliminates the necessity of heating this should proceed to the secondstage and' become thoroughly digested, i. e. cease evolving gases andother decomposition products.y However, by elutriating such supernatantfrom the first stage containing partially digested solids tending toremain in suspension and in the scum, I ilnd that sludges settled fromthese elutrlated solids require but little ferric or aluminum coagulantand dewater easily in vacuum filters, producing a illter cake which ispractically odorless.

Figure 1 graphically shows this discovery, and Figure 2 illustratesdiagrammatically a sewage treatment system including digestion, and theapplication of the present invention thereto.

Thecurves or graphs Nos. I to 4 of Figure 1 result from plotting variousferric chloride additions (horizontally measured) against the resultingfilter yields (vertically measured) in pounds of dry sludge solids inthe yielded illter cake per square foot of filter surface per hour ofilltration time. The horizontally plotted ferric chloride additions showthe poundsA of anhydrous ferric chloride used per pounds of dry sludgesolids present in the yielded lter cake, i. e. the percentage of ferricchloride used on dry cake solids.

The sludge of graph No. I was taken from the second stage of a`digestion system operating on mixtures ol primary dsettled raw andsecondary settled activated sludge, and the sludge produced for graph No. 2 was taken from the dirty supernatant of the'flrst stage digestiontank of the same digestion system.: The digested sludge for graph No. Iwas taken from the bottom of the second stage and, therefore,represented digested sludge. The sludge for obtaining graph No, 2 wasproduced by takingthe'dirty supernatant or sludge liquor from the top ofthe ilrst digestion sludge from the bottom of `digestion tank No. 2

and subjecting these successive batches to ltration under vacuum andmeasuring the filter yields. Graph No. 2 resulted from using thequestion.- The resulting graph would not be same procedure on successivebatches of the,

sludge produced from the upper or supernatant lmixture from digestiontank No.I. Comparison of the two resulting graphs Nos. I and 2 clearly,shows that the dewatering of the sludge solids in the supernatantliquor was not only definitely slower than the dewatering of the solidsin the digested sludge of graph No. I, but considerably more chemicalwas required in the ca se o'f graph No. 2 to produce the same filtrationrates recorded in graph No. I. For example, to obtain one pound 4filteryield in both instances, about 6.5 percent ferrie chloride (6.5 poundsper 100 p ounds of vdry cake solids) was necessary with the digestedsludge of graph I, and 9.5-percent ferrie chloride for the sludge madefrom the supernatant liquor in graph 2, meaning that the llatter sludgerequired 46 percent more ferric chloride than did the digested sludge toproduce the same filter yield of one pound of cake solids per squarefoot of filter area per hour.

When portions of the sludges represented by graphs I and 2 wereelutriated by the countercurrent method, using about 3 'volumes of thepurified treatment plant eiliuent as elutriating water to one volume ofthe digested sludge of practically depicted on Figure 1, because thefilter cake yields, at all ferric chloride additions, were below theminimum allowable limit of one pound per square foot per hour. However,by elutriating such mixtures from the same digestion tank, n ibst of thesolids in the supernatant were washed free of gases and other. solubledecompositlons products and settled with the washed solids of thedigested sludge to a new and denser sludge. The resulting filtrationgraph was practically identical with graph 3. v l

These tests also showed that with elutriation of the dirty supernatantfrom the first stage of digestion, most of the solids that normallyfloat and form a dark s cum over the dirty supernatant also lost theirmatted or trapped gas bubbles that produce the notation and settled tothe bottom of the elutriation settling tank, thus materially improvingthis bothersome trouble common to some digestion tanks.v

(d) I have also found that, whereas the soluble decomposition productsconcentrated sludge moisture or liquor during sludge digestion, formgelatinous precipitates with ferrie and aluminum salts added to renderthe sludge solids graph No. I, and the dirty supernatant that made thesludge for graph No. 2, graphs 3 and: 4 resulted. Graph: No. '3 resultedfrom the elutriation of the digested sludge ofl graph No. I and graphNo. `4 resulted fromthe elutriation of the dirty supernatant thatproduced the sludge of graph No. 2. In the latter graphs, 3 and 4, thegreat difference in chemical demand exhibited coarsely coagulatedanddrainable in modern mechanical filters and thus detrimentallyinfiuence economical filtration, these same decomposition products maybe used in conjunction with small amounts of precipitatng chemicalagents to aid in the clarification of the diluted sludge supernatantduring the hereinbefore de tailed methods` of elutriation. The solubledecomposition products that accumulate in the sludge liquor duringnormal sludge digestion and form precipitates with ferrous, ferric andaluminum salts are principally bicarbonates'of ammonia, ammoniumcompounds of carbonates,

bythe sludges of graphs I and 2 has almost disappeared'tandboth of theelutriated sludges become easilygdrainable in vacuum filters atremarkably low chemical coagulant additions. For example, to againobtain the one pound filter yield on the elutriated digested sludge ofgraph No. 3, about 1:8 percent ferric chloride isused,

i. e. 72 per centI less than in graph I and for the elutriated solids ofthe supernatant sludge in graph l, 2 percent ferric chloride was used,which v is 79 percent less than -the 9.5 percent in graph 2.

Whereas the chemical addition, to the sludge of graph 2 was 46 percentgreater than for the sludge of graph I to obtain the equivalent minimumfilter yield of one pound after elutriating the solids in the samesludges, this difference in' ferric chloride'additions is `but tenpercentof a much smaller addition, which latter difference is almostnegligible from a cost standpoint, namely, the difference in costbetween a 1.8 percent and 2 percent addition on the one hand and 6.5percent and 9.5 percent addition on the other-hand. The

cake solids and the former is but 0.2 percent ferric chloride on cakesolids, making the difference between graphs I and 2 fteen times greaterthan the dierence between graphs 3 and 4.

In attempting to filter mixtures of the supernatant and digested sludgeas they came from the rst stage digestion tank, the chemical additionswere so high and the lter yields so low, due to the mixture containingbut about one percent solids, that economic filtration was out of thelatter difference is 3 percent ferric chloride on carbonate carbamate,and fatty acid compounds and numerous allied decomposition products.

In the vforegoing paragraph, there is a distinction made o n the onehand between the Aclarification of a turbid liquid by flocculation inthe process of sedimentation wherein the liquid being purified isrelatively stationary and the chemically formed precipitate floccules'move through same by settling, and, on the other hand, clarification byfiocculation or coagulation in the process of filtration through astationary medium, wherein the medium and accumulating bed of flocculesis relatively stationary and the liquid being purified movesl throughthe same. Whereas gelatinous fioccules may readily settle by zgravityto,` a relatively loose sludge through any amount of stationary waterand leave a clear supernatant liquid, attempts to reverse the process,when using the same amount of the same flocculating agent, by passingthe liquid through a relatively tightly compacting stationary cake ofgelatinousfloccules by gravity or aug- Imented pressure, lsoon producesan impervious gelatinous, compacted scum of floccules 4thus rapidlypreventing further percolation of clear liquid. This is particularlytrue in the clarification of sludge supernatant and mixtures of samewith digested sludge. Clarification of supernatant land its mixtureswith sludge by occulation during elutriation-sedimentation, when usingsmall quantities of floc forming chemicals is particularly easy.However, attempts to reverse the process by passing any appreciablequantity of liquid through the compacting stationary bed of flocculesand sludge solids by filtration rapidly ends in cessation offiltrationunless (a) exceedin the' ingly large quantities of flocforming chemicals are added, or (lr) the excessive amounts ofarnmoniacal bicarbonates and such present in the sludge liquor and whichform excessive gelatinous flocs with the added flocculating agents arefirst displaced from the sludge liquor by purer water. In other words,these ammoniacal compounds may be of aid in using clarification byfiocculating and sedimentation and of detrimental infiuence when usingclarification by flocculation and filtration in .any mechanicalfiltration equipment havinga finely porous cloth or other felted orrigid filter medium to intercept and hold back all gelatnous'iiocculesthrough which the clear liquid or water is passed.

For example, commercial aluminum sulfate or crude alum is excellent forraw water fiocculation and clarification purposes by sedimentation.However, it cannot be used economically to pre-` cipitate out all of theammoniacal bicarbonates and such and to coagulate the gelatinous solidsin digested sludges so that draining of the clear water through afiocculated and coagulated massl of cake produced in a vacuum filterresults. However, alum can be successfully used for filtration of suchsludges if the latter are first elutriated to i remove the excessiveamounts of carbonates and bicarbonates and such that form too muchgelatinous aluminum hydroxide precipitate with the added alum. Thisgelatinous precipitate of aluminum hydroxide in excessive quantitiesrapidly blinds any filter medium, thus rendering the use of alum forthis purpose of no practical value.

ing and sweeping down action of suspended sludge particles in the dilutemixture.

The lime hydrate added in this procedure forms, on the one hand,insoluble lime carbonate precipitate with the ammoniacal .and otherbicarbonates and carbbn dioxide gas present in the sludge liquor, and.on the other hand, soluble ammonium hydroxide. This liberated hydroxidecan also unnecessarily consume ferric and aluminum coagulants to formgelatinous hydroxide precipitates when the sludge is coagulated withsuch salt solutions. These gelatinous preoipitates can retard filtrationrates. 'I'herefore in using this procedure with lime as oxide orhydroxide, it is better to proceed as follows: (l)

`add the lime to the supernatant liquor either alone or with theelutriating water, or followed by the addition of the elutriating water,(2) mix all4 three, (3) allow the sludge solids and carbonateprecipitate to settle to'a sludge, (4) decent the diluted elutriatecontaining most of the ammonium hydroxide and other dissolved impuritiesand (5) simultaneously or adlibitum withdraw and dewater the resultingsludge. In most iiistances further economy in coagulants for dewateringthis sludge will be effected by again elutriating the sludgeinrelatively purer water -to remove as far as possible the remainingammonium hydroxide. This procedure can be eifectively used with thecounter-current elutriation method disclosed in my Patent No. 1,999,973.

However, in the clarification of the elutriate supernatant by thereverse process of sedimentation, small quantities of alum solutionadded to the purer elutriating water show remarkable results. Theprecipitate of aluminum hydroxide formed by adding any aluminum salt tothe dilute water and sludge supernatant and/or its .mixtures withdigested sludge containing ammoniacal bicarbonates and such, rapidlygathers into iloccules which are gelatinous and possess positive gel orcolloidal surface properties within the pH ranges common t'o digestedslu-fdge liquors in their normal and more diluted concentrations. Thefloccules having positively charged surface properties rapidly settlethrough the dilute mixture and adsorb the negatively charged, finelydispersed suspended sludge particles, thus causing their removal withthe result that the elutriated or diluted mixture rapidly clarifiesitself producing a very clear'supernatant elutriate and a rapidlysettling elutriated sludge in the tank or tanks used for elutriationsedimentation. The overflowing or decanted clear supernatant orelutriate can then either flow back to the sewage treatment process or,if pure enough, be conducted to the plant eiiluent as shown in Figure 2without proceeding through the treatment processes. The alum solutioncan be added either to the elutriating water first or to themixturetriation sedimentation if desired.

Small additions of lime hydrate may. also r'be used for the purpose ofprecipitating the carbonates and bicarbonates in the 'diluted super-However, calcium hypochlorite in small quantities is better than lime ifprecipitating action c'ombined with purifying or sterilizing action isdesred. The liberated calcium hydroxide likewise precipitates as acarbonate, etc. with the soluble ammoniacal decomposition products whilethe liberated hypochlorous acid destroysorganisms and any unstableorganic compounds or matter. The liberation of hypochlorous acidforreducing the biochemical oxygen demand of the supernatant elutriatein the elutriation system can be further facilitated with the incidentalformation of a positive hydroxide fioc by making a milk of calciumv orother hypochlorite and adding to the same either a weak solution ofaluminum or ferric natant of relatively low biochemical oxygen' delmand.

It is evident from the foregoing that the elutriation of the finely andcoarsely suspended solids in supernatantliquors from open and closedseparate digestion tanks iand/or the elutriation of suchsolids inunsettled mixtures sludge liquor and solids from the first or subsequentstages of multi-stage digestion tanks accomplishes threedeiiniteadvantages and novel functions in 4the separation of digestion tankcompanied by mechanical flocculation before elunatant. A rather granularprecipitate of lime i carbonate results which exercises someenclosliquor from the digested or partially digested solids, togetherwith other advantages, i. e., (1') provides a simple and moresatisfactory separation of suspended digested and partially digestedsolids from a clearer more dilute sludge liquor, (2) lessens thenecessity of large digestion tank space `by eliminating the necessity ofcopious second and third stage digestion space in those cases where suchspace is principally employed for bf digested and (f) scum forming tlingsaid solids' from said mixture of water` and I ing to sludge dewatering.

y Further advantages are, (ai elimination of the use of extensive sandbed areas-for further clarifying the supernatant liquors, (b) dewateringof the. elutriated supernatant solids and sludge solids, together withsmall coagulant additions in the same vacuum lter equipment withoutincreasing the lter area, (c) more freedom from .turbulent conditions indigestion tanks, (d) shorter digestion periods can be used, whichfalsoprovides more digestion space, (e) the use of stirring mechanisms tohasten digestion can be freely liquor in a tank other than the primarysettling system, and dewatering said solids. d

2. In a sewage treatment system having primary settling and sludgedigestion.. the process of eliminating fromethe primary settling anddigestion systems solids suspended in the sludge digestion liquor .whichcomprises withdrawing a supernatant liquor from the digestion system,elutriating the solids suspended in said liquor with water relativelypurer than vsaid liquor .to remove gases and other decompositionproducts from said suspended solids so that they will settle and"settling said solids from said mixture of water and liquor in a tankother than the primary 'settling system, and dewatering said solids, and

returning the elutriate to the treatment system.

3. In a sewage treatment system having .pri,

. mary settling and sludge digestion, the process used withoutnecessitating copious sedimentation y space for sludge and clearersupernatant liquor, separation in subsequent digestion tank spacetroubles in digestion tanks can be lessened if 'not eliminated. In fact,elutriation treatment of sludge mixtures* or of hastily separated`dirtysupernatant obtained -by rapid sedimentation treatment of such'stirred mixtures makes possible amore widespread use of such stirringdevices by and/or eliminating expensive second or subsequent stagedigestion space.l

Referring' further to Figure 2, which illustrates diagrammatically atypical sewageI treatment plant including a-two-stage digestion-systefor the primary settled sludge and/or secon ry sludge, it will be notedthat the supernatant may be removed from either one or both digestiontanks and delivered to the elutriation system with ,or without digestedsludge. By reason of the present invention,

natant and the mixture delivered to the elutriation system. It-willfurther be noted that the occulating and/or purlf to the .elutriationSystem at any desired poin't` before elutriation sedimentation. -Insmaller plants, a'singleelutriation tank may be used as hereinbeforedescribed. .In larger plants, the use, of two tanks, as indicated inFigure 2,' may be employed. The decanted/or overiiowing elutriate fromthe elutriation system is returned to primary sedimentation 0r. ifilocculating and/or purifying agents 'are used in the elutriationsystem, the elutriate can be discarded'with the treatment planteiiiuent.The elutriated solids from the'supernatant liquor and/or mixtures ofthis. liquor with digested sludge are conducted to the sludgedew'atering system to be disposed of as by dewatering and/orincineration. It'will thus be seen that these solids p ass directly outofthe primary and digestion treatment systems and go directly to sludgedisposal.

' Iclaim:

' 1. In a sewage treatment system having primary settling and sludgedigestion,- the process of eliminating `from the primary settling anddigestion systems solids suspended in the sludge digessuch mechanicallystirred the digested sludge from either tank of the system may be mixedwith the superportions in order to remove gases and other deagent can beadded gestion liquor which y supernatant lliquor from the digestion filitriating water and vof eliminating from the primary settling anddigestion systems solids suspended in the sludge digestion liquor whichcomprises withdrawing a mixture of partially and more completelydigested solids and their associated supernatant liquor from thedigestion system, elutriating the solids suspended in said liquor withwater relatively purer than said liquor to remove gases and otherdecomposition products from said suspended solids soV .that thgywiilsettle and settling said l solids from said mixture of water and liquorin a tank other than the primary settling system, and dewatering saidsolids.

4. In a sewage treatmentI system having!)I pri? mary settling and sludgedigestion, the process of eliminating from the primary settling anddigestion systemssolids suspended in the sludge digestion liquor whichcomprises withdrawing diof the digestion tion o1' Athe digestion system,mixing the withdrawn portions with water-relatively purer than thesolids associthe sludge liquor to elutriate ated with the digestionliquor in the withdrawn `composition products from said solids so thatthey will settle, and settling said solidstrom said mixture in a tankother systemand dewatering said solids. 4 5. In a sewage treatmentsystem having primary settling .and sludge digestion, the process ofeliminating' from the primary settling and digestion systems ,solidssuspended in the sludge'di comprises withdrawing the system, forming inthe diluted mixture aluminum hydroxide precipitate ocs with the .solublebicarbonates present in said sludge liquor by diluting and mixing saidliquor with relatively pure elua solution of an aluminumsalt andsettlingthe solids and -tlocs in a tankother than the primary settling system,toform a -iioced Super' x culated elutriatedJ sludge and a clariiflnatant liquid, removing the supernatant liquid,

tion liquor which comprises withdrawinga super- '70 natant liquor fromthe digestion system, -elutriating the solids suspended in said liquorwith water relatively purer than said liquor to remove gases and otherdecomposition products from said suspended solids so that they willsettle and setand dewatering said sludge. v

' ,6. In' a sewage treatment system having' pri' mary settling andsludge digestion, the process o! eliminating vfrom the primary settlingand diges;

supernatant liquorironi the digestion system-,j

dnuting and mixing saidliquor with relatively pure elutriating water anda solution of'a saltv capable o; forming inthe diluted. mixture iiyedroxide precipitate iios with the soluble bicarthe .primary settling 1 0attacca A' bonates present 'in said sludge liquor and iorming such ilocsand settling the ilocs and solids in a tank other than theprlmary'settling system, 'I to form a iiocculated elutriated sludge anda clarined supernatant liquid, removing the supernatant liquid, anddewatering said sludge.

7. In a sewage treatment system having primary settling and sludgedigestion the process of eliminating from the primary settling anddigestion systems solids suspended in the sludge diges tion liquor whichcomprises withdrawing the supernatant digestion liquor from thedigestion system, diluting and mixing said liquor withv relatively purewater and a chemically active purifying agent capable of reducing thebiochemical oxygen demand of the diluted vmixture and settling thesuspended solids in a tank other than the primary settling system, to anelutriated sludge, removing the supernatant liquid, and de' watering thesludge.

8. -In a sewage treatment system having pri- 'mary settling and sludgedigestion the process of eliminating from the primary settling anddigestion systems solids suspended in the sludge digestion liquorA whichcomprises withdrawing the` supernatant digestion liquor from thedigestion system, diluting and mixing said liquor withrelatively purewater and a chlorinating agent in amount capable of reducing thebiochemical oxygen demand of the diluted mixture and settling thesuspended solids in a tankother than the primary settling system, to anelutriated sludg removing the supernatant liquid, and dewatering thesludge.

9. In a sewage treatment system havingslludgedigestion, the steps ofwithdrawing the supernatant digestion liquor, elutriating the solidssus- .pended in said liquor, returning the supernatant liquor to primarysettling, settling the solids and dewatering the sludge.

10. In a sewage treatment having sludge digestion, the processcomprising elutriating the solids contained in the supernatant digestionliquor and, the digested solids, returning the supernatant elutriate toplant eiiluent, settling the solids and dewatering the sludge.

1l. In a sewage treatment'system having primary settling and sludgedigestion the process of eliminating from the primary ttling anddigestion systems solids suspended in the sludge digestion liquor whichcomprises withdrawing the supernatant digestion liquor from thedigestion system, diluting and mixing said liquor with relatively purewater `and a chlorinating agent and a salt capable otforming in thediluted mixture gelatinous hydroxide precipitate iiocs with the solublebicarbonates present in said sludge liquor and settling the ilocs andsuspended solids in a tankother than the primary settling system, to asludge, and dewatering the sludge.

12. In a sewage treatment system having primary settling and sludgedigestion, the process ot eliminating from the primary settling anddigestion systems solids suspended in the sludge digestion liquor andammoniacal bicarbonates dissolved in the same which compriseswithdrawing the digestion liquor from the digestion system, addingcalcium hydrate and water relatively purer than said digestion liquor todilute l said' liquor, mixing said limehydrate, liquor and water so thatthe soluble ammoniaca] bicarbonates and dissolved carbon dioxide gaspresent in said sludge liquor will precipitate as lime carbonate andliberate ammonium hydroxide in the diluted mixture, settling said limecarbonate precipitate and said suspended solids in the diluted mixturein a tank other than the primary settling system, decanting thesupernatant dilute solution to remove the'soluble ammonium hydroxide andother impurities from said solids and dewatering the solids.

ALBERT L. GENTER,

